Methylcobalamin is a cobalamin (MeB12) used in peripheral neuropathy, diabetic neuropathy etc. It is a form of vitamin B12. This vitamer is one of two active coenzymes used by B-12 dependent enzymes in the body, and is specifically the B-12 form used by 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), also known as methionine synthase.

Methylcobalamin is notable for being one of the few examples in nature of a bona fide organometallic bond. Methylcobalamin has been studied in conjunction with sleep-wake rhythm disorders, where it appears to yield benefits, but at a low or inconsistent level. It is used in treating diseases of vitamin B12 deficiency (such as pernicious anemia), or diseases of effective B12 deficiency, such as vitamin B12 metabolic pathway pathologies. One study suggests that once absorbed, methylcobalamin may be retained in the body better than cyanocobalamin.